The invention relates to the handling of liquids in industrial and commercial processes. More specifically the invention relates to filling of tanks with liquids. One example of a case where tanks must be filled is the filling or refueling of a motor vehicle which is powered by a fuel which is dispensed to the vehicle in liquid form.
Any liquid expands when it is warmed. Consider a completely closed tank which is nearly filled with a liquid. Suppose the tank is warmed, for example by the sun shining on it. The liquid in the tank expands, and may come to completely fill the available internal volume of the tank. If there is further warming, and the liquid has no further available space within the tank into which it can expand, the liquid develops extremely large forces against the tank walls and the tank may split apart, releasing the liquid in an uncontrolled manner. Such a release is obviously undesirable, especially if the liquid is toxic or flammable.
Every liquid has associated with it a "vapor pressure" which is a function of temperature. The phrase "vapor pressure" has a very specific meaning well known to those skilled in the arts of chemistry and chemical engineering. It is convenient to divide liquids into two classes:
1. Class 1 liquids are those liquids which have a vapor pressure below atmospheric pressure, at all normal outdoor temperatures. PA1 2. Class 2 liquids are those liquids which have a vapor pressure above atmospheric pressure at any outdoor temperature. PA1 a. A mechanical valve is permanently installed inside the tank. The valve is on the inlet line or feed line. This valve senses the liquid level and closes when the correct liquid level has been reached in the tank. PA1 b. A liquid level sensor can be placed inside the tank. When the correct liquid level is reached, a signal is sent to a controller, which in turn stops the flow of liquid to the tank. PA1 c. To quote from "Handbook--Butane-Propane Gases", Third Edition, 1942, page 104, in a method which is applied to "tank trucks", "there is a fixed outage tube in each tank, that extends from the top of the shell to the correct point to indicate when loading is finishing, this tube having a valve through which vapor will vent until the liquid reaches the bottom of the tube. The valves are then shut and the liquid and vapor hose disconnected . . . ". PA1 d. If the tank is removable from its usual place of use, and if it is not too large, it can be placed on a scale during filling. The maximum allowable weight of tank and contents is known. The weight of the tank and contents is observed on the scale during the filling operation. When the correct weight has been reached, the filling process is stopped. PA1 a. The mechanical valve could malfunction, and allow a larger than correct amount of liquid to enter the tank. It would be very difficult for the user of this method to be aware that tanks are being overfilled. PA1 b. Some sensors would require an electrical connection to the tank. To make this connection each time the tank is filled adds to the complexity of the filling process, and to the hazard, if the liquid being handled is flammable. Again in case of malfunction, the tank might be overfilled without the user being aware of it. PA1 c. The method as described in the reference does not provide automatic operation and therefore is only suitable for use with a trained operator in attendance. In filling of large tanks, which is the subject under discussion in the quoted reference, the feed rate is small relative to the tank volume and there is time for the operator to take action to stop the fill. In filling smaller tanks where the total filling time may be only 1 to 2 minutes, a delay of even a few seconds could result in an overfilled tank. Therefore the described manual method could not be used. PA1 d. This method is only applicable to relatively small tanks, and to tanks that can readily be removed from any equipment with which they are used. PA1 an attempt to fill a tank which is already correctly filled PA1 sensor failure
Vapor pressure is an intrinsic property of a given liquid at a given temperature and can be thought of as an outward force exerted on the surroundings, by the liquid.
Atmospheric pressure is typically in the area of 101.325 kiloPascals, or 101,325 kPa. This pressure can also be expressed as 14.696 pounds force per square inch absolute or 14.696 psia.
Consider a warm summer day when the outdoor temperature is 30.degree. C. (86.degree. F.). An open dish containing a Class 1 liquid is placed outdoors. Examples of Class 1 liquids are water and gasoline. The Class 1 liquid in this experiment evaporates in a relatively gradual process.
Now consider the same experiment, this time performed with a Class 2 liquid. The Class 2 liquid would be seen to be boiling and in a relatively short time all the liquid would have disappeared. Depending on the liquid used in the experiment, the boiling might be extremely rapid and violent, or might be relatively slow.
A Class 1 liquid can be stored in a tank which is not hermetically sealed. The tank can be essentially open to the atmosphere, and the pressure inside the tank can be essentially equal to atmospheric pressure. On a warm day some of the liquid may evaporate and some of the vapor may escape from the tank. But the bulk of the liquid remains in the tank. If the liquid expands and completely fills the tank, on further warming excess liquid can escape from the tank, because the tank is not hermetically sealed, rather than building up a large force inside the tank.
On the other hand, a Class 2 liquid must be stored in a tank which is completely closed and hermetically sealed. If there was any communication with the general outdoor environment, the liquid in the tank would boil off and after a time all the liquid in the tank would be lost. It follows then that if there is thermal expansion of the liquid, there must be adequate space within the tank. If the tank becomes completely full with liquid, and if there is then further warming, since there is no way for the liquid to escape very large forces are developed and the tank will split open.
To avoid disasters that could result from splitting of tanks in this manner, the following procedure is used and is well known to those skilled in the art:
When filling tanks with Class 2 liquids, the tank is only partly filled. Enough space is left free of liquid so that in any subsequent warming process, such as due to the sun shining on the tank, thermal expansion of the liquid still does not result in the internal volume of the tank being completely full of liquid. There is still some space in the tank that is free of liquid.
The remainder of the present document deals only with Class 2 liquids. All further reference to "liquid" or "liquids" should be understood to mean Class 2 liquids, as defined above.
When a Class 2 liquid is in a completely closed and hermetically sealed tank, in general there is a portion of the tank which contains liquid, and there is another portion of the tank which does not contain liquid. In the latter portion of the tank is found vapor which has been produced by boiling of the liquid. In other words, there is a vapor space in the tank and a liquid space. Both spaces contain the same chemical substance. In one case the chemical substance is present in the liquid form, and in the other case the same chemical substance is present in the vapor form. There is no air in the tank.
The pressure in the tank is equal to the vapor pressure of the chemical substance, at the temperature of the tank. If the tank is warmed, for example by the sun shining on it, the contents of the tank will be warmed. The vapor pressure, an intrinsic property of the chemical substance, increases with temperature, and therefore the pressure in the tank will increase. As long as there is still some vapor in the tank, the increase in pressure in the tank is relatively moderate and the tank can contain the increased pressure.
When both liquid and vapor coexist in the tank, both the liquid and vapor phases are at the same temperature and pressure, or very nearly at the same temperature and pressure. The two phases are said to be in equilibrium with each other. If there is the slightest warming, additional liquid boils off. If there is the slightest cooling, some vapor condenses into the liquid. Boiling of the liquid is incipient, and condensation of the vapor is incipient. The liquid and the vapor are each said to be "saturated". Handling of saturated or nearly saturated liquid creates difficulty, because of the ease with which vapor is formed. For example, vapor can form in pumps conveying saturated liquid, which may slow the pumping action and/or damage the pump.
The terms saturated liquid and saturated vapor are standard and accepted terms well known to those skilled in the art of chemical engineering.
The situation inside a tank containing a Class 2 liquid, with the vapor phase and the liquid phase in a dynamic equilibrium with each other, and with no air present, is a situation which is not met in everyday life. Failure to understand the behavior of this type of system is the root cause of the Three Mile Island Nuclear Power Plant disaster in 1979. Also in 1979 there was a railroad accident in Mississauga, Ontario, involving cars containing chlorine. Failure of the authorities to understand the behavior of a Class 2 liquid resulted in hundreds of thousands of people being unnecessarily kept away from their homes, and thousands of businesses being unnecessarily closed, for a lengthy period. The behavior of the vapor and liquid phases inside a tank containing a Class 2 material must be fully understood in order to understand the apparatus and method of the present invention. Especially, it must always be kept in mind that there is no air inside the tank.
In industrial practice, typically there is a supply tank from which liquid is drawn. This liquid is moved by pump or by other means to a tank which is to be filled. While the tank to be filled may initially be essentially empty, there usually would be some liquid and therefore some vapor in the tank. When at least a portion of the internal volume of the tank is occupied by vapor, it is possible to force further liquid into the tank, which results in vapor in the tank being condensed into the liquid phase in the tank. However in order to proceed more easily and more rapidly with the filling process, vapor from the tank to be filled can be returned to the supply tank, during the filling process. The vapor flows from the tank being filled, to the supply tank, via a "vapor return line".
The volume of vapor being returned is essentially equal to the volume of liquid entering the tank which is being filled. This condition defines a true or full-fledged vapor return system.
The apparatus and the method of the present invention rely on and require use of a very small flow of vapor from the tank being filled, back to the supply tank, during the filling process. The volume of vapor which returns to the supply tank is a very small fraction of the volume that would return in a full-fledged vapor return system.
This small flow of vapor can be referred to as a bleed flow or auxiliary flow. It can be regarded as a signal flow, or the small-diameter line which carries this small flow can be regarded as a signal line which carries information about the liquid level within the tank being filled.
From the previous discussion of thermal expansion of liquids, it is obvious that when filling the completely closed and hermetically sealed tanks which are used with the subject liquids, namely Class 2 liquids, extreme care is needed to stop the filling process at the correct point, so as to avoid overfilling of the tank. An overfilled tank may still have some vapor space in the tank, i.e., space which is free of liquid, but this space is not enough to accommodate possible future thermal expansion of the liquid.
The maximum allowed amount of liquid in a given tank can be expressed as a percentage of the total internal volume of the tank. The allowed percentage is typically in the range of 50 to 80% of the internal volume of the tank. The exact value depends on the liquid being handled. At the time a tank is filled, if the liquid that is being fed to the tank is unusually cold, the allowed percentage, otherwise referred to as filling ratio or filling density, properly should be even less than the normal value.
Various methods are currently in use to stop the process of filling a tank with liquid, at the correct point, so that the tank is not overfilled. These methods include:
There are various disadvantages and deficiencies to the above methods, including:
Any method which utilizes a valve or other mechanical equipment on the liquid feed line suffers from adverse effects of contaminants in the liquid feed. Because all the liquid goes through said valve or other mechanical equipment, contaminants tend to build up and in time cause a malfunction.
In view of these deficiencies, the various industries which deal with Class 2 liquids, i.e., liquids which must be handled and stored in a saturated or incipiently boiling state, are seeking improved methods of controlling the filling of tanks with these liquids. The ideal control method would have the following attributes:
The control method would automatically stop the fill at the correct point, without supervision by human operators or observers, and would automatically take into account normal and abnormal operating conditions.
The control method would be self-supervising so that in case of malfunction of one of the components of the control system a warning is given and the system automatically shuts down. If the malfunction is such that the tank currently being filled may be overfilled, a warning to that effect is given. In any case, the control system automatically refuses to fill further tanks until repairs have been made and the system has been reset by authorized service and repair personnel.
The equipment required to put the control method into practice would not be unduly expensive, and the use of the equipment would not complicate the tank filling process.
Usually the number of tanks to be filled is relatively large. Therefore to put the control method into use the component(s) required on or in each tank should in particular be very simple and inexpensive and should require little or no maintenance.
Any hardware should not have to handle all the liquid which is supplied to the tank, and therefore, because the hardware is handling relatively little liquid, there would be a reduced tendency to suffer malfunctions due to contaminants in the liquid.